The technical portion of the profession of an optician consists in mounting a pair of correcting ophthalmic lenses on a rimmed eyeglass frame as selected by a wearer. Such mounting comprises three main operations:                acquiring the shape of the internal outlines of the surrounds of the frame;        centering each lens, which operation consists in positioning and orienting each lens appropriately in front of each eye of a wearer; and then        machining each lens, which consists in cutting out or shaping its outline to the desired shape, taking account of the shapes of the surrounds and of the defined centering parameters.        
In the context of the present invention, attention is given more particularly to the first and third operations referred to as acquisition and machining. The specific object of the optician is to edge the ophthalmic lens in such a manner as to enable it to be fitted mechanically and pleasingly to the shape of the corresponding surround of the selected frame, while also ensuring that the lens performs the optical function for which it is designed as well as possible.
With rimmed frames, the machining operation includes in particular a bevelling step that serves to form an engagement ridge, commonly called a bevel, on the edge face of the lens and suitable for engaging in a groove, commonly called a bezel, that runs along the inside face of the corresponding surround of the frame.
Both the acquisition and the machining operations need to be performed with particular care so as to ensure that the lens can be properly engaged in its surround, without force, and at the first attempt, i.e. without requiring a subsequent reworking.
In order to acquire the shape of the bezel, it is general practice to use an outline reader appliance that includes a feeler that picks up the shape of the bezel. Nevertheless, at the end of this feeling operation, errors are observed in the measurement of the shape of the outline. These errors are inherent to the reader appliance that may present resolution that is not sufficient, or assembly defects, or indeed that may be damaged or out of adjustment. In addition, while the bezel is being felt, any deformation of the frame (as a result of the feeler bearing against the bezel) likewise give rise to errors.
At the end of the machining operation, edging errors are also observed, such that the actual shape of the edge face of the lens does not correspond exactly to the desired shape. These errors are likewise inherent to the shaper appliance that may present resolution that is insufficient, or assembly defects, or that may include a grindwheel that is worn in shape. Furthermore, the bending deformations of the lens (due to the grindwheel bearing against the edge face of the lens while it is being machined) also give rise to errors, as do the phenomena of lenses expanding while they are being machined.
To sum up, and given the various errors and inaccuracies, a lens as machined in this way presents an outline that rarely corresponds exactly the outline of the bezel of its surround. It runs the risk of being either too big, thereby constraining the optician to perform additional and time-consuming machining of the bevel, or too small.
In order to increase the yield of lenses that are correctly edged at the first attempt, it is known to correct the defects of acquisition and shaper appliances in such a manner as to increase their resolutions and so as to enable them to take a greater number of parameters into consideration. It is also known to calibrate the appliances frequently. Nevertheless, such methods are lengthy, complex, and expensive to implement. Furthermore, the parameters actually taken into consideration are not exhaustive. As a result, the yield of lenses that are correctly edged at the first attempt is still not satisfactory.
Furthermore, a large fraction of lenses that are considered as being mountable in their surrounds are in fact slightly too big relative to their surrounds, such that once they have been engaged therein, they are mechanically under stress. As a result, such lenses are weakened and their treatment layers are likely to be damaged more quickly. Furthermore, these mechanical stresses modify the optical characteristics of lenses to some extent and that can be troublesome for their wearers.
It is also known to acquire the shapes of the bezels of the surrounds of an eyeglass frame by means of a database registry containing a plurality of records, each associated with a particular model of eyeglass frames. Nevertheless, as a result of manufacturing dispersions, it is observed that no two eyeglass frames of a given model ever present exactly the same shape. Consequently, the shapes acquired from the database are generally slightly different from the real shapes of the bezels of the particular eyeglass frame as selected by the wearer. As a result, lenses machined as a function of such acquired shapes are not always mountable in the surrounds of the selected frame, such that it is often necessary to rework the machining of their engagement ridges.
It is also known to acquire the shape of the bezel of one of the surrounds of an eyeglass frame as a function of the shape previously acquired for the bezel of the other surround of the same eyeglass frame. Nevertheless, as a result of manufacturing dispersions, it is observed that the two surrounds of an eyeglass frame are never completely symmetrical. Consequently, the shape of a bezel as derived in this way is generally slightly different from its real shape. As a result, a lens machined as a function of such a derived shape is not always mountable in the corresponding surround of the frame, such that it is often necessary to rework the machining of its engagement ridge.